Packing modes in some mono- and disubstituted phenylpropiolic acids: repeated occurrence of the rare syn,anti catemer.

The catemer is an infinite one-dimensional pattern formed by the carboxylic acid group in crystals, and is constituted with O-H...O hydrogen bonds. The catemer is uncommon and may be contrasted with the ubiquitous carboxylic acid dimer, the favored mode of association of this functional group. Both catemers and dimers, however, have two O-H...O hydrogen bonds for each carboxy group, so the reasons for the rarity of the catemer must lie elsewhere. In this paper, we describe a group of around 25 phenylpropiolic acids in which the catemer is the default packing mode. Exceptionally, the particular catemer that is found in this family is of the very rare syn,anti variety. We show that a necessary ingredient in catemer formation is a supportive C-H...O hydrogen bond from a proximal C-H group, which is located on the phenyl ring, ortho to the ethynyl group, and suitably activated by electron withdrawing substituents. When steric factors become noteworthy, alternative patterns are adopted, such as the syn,syn catemer and, in one case, a rare cisoid dimer. When electron-donating groups, either through inductive effect such as methyl or through resonance such as halogens, are present on the phenyl ring, the dimer is formed in all but one case. Polymorphism seems not to be an issue in these carboxylic acids in that no compound would generally crystallize as both a dimer and a catemer. It may be concluded that a supporting interaction, in this case a C-H...O hydrogen bond, is the essential condition for the formation of any carboxylic acid catemer. Catemers are so rare because it is difficult to set up this type of supporting interaction in most carboxylic acids.